1. Field of the Invention
This invention relates to optical transmission systems and more particularly to variable optical attenuators (VOAs) within optical transmission systems. Even more particularly this invention relates to circuits and methods for automatically adjusting variable optical attenuators and to methods and systems for calibrating variable optical attenuators to allow for the automatic adjustment of the variable optical attenuators.
2. Description of Related Art
Variable optical attenuators (VOA) are well known in the art and permit the attenuation of optical or light signals as they are transferred in an optical transmission system. Variable optical attenuators are of two fundamental types, mechanical and non-mechanical. The mechanical variable optical attenuators have moving parts such as stepper motors to adjust an optical filter to vary the attenuation. In non-mechanical variable optical attenuators, the mechanism employed to adjust the attenuation is either a magneto-optic effect or thermo-optic effect that modifies the light waveguide. The attenuation settings of a non-mechanical variable optical attenuator are generally wavelength dependent. Mechanical variable optical attenuators on the other hand provide adjust the optical attenuation in a manner that provides relative independence of wavelength.
Mechanical variable optical attenuators such as described in U.S. Pat. No. 6,149,278 (Mao, et al.) have a pair of substantially parallel mirrors that attenuate an optical signal based, at least in part, on the rotation angle of the mirrors. When the pair or mirrors is in a predetermined position, an input optical signal is directed from an input port to an output port with a minimum insertion loss. As the pair of mirrors is rotated, the optical signal is shifted in a parallel fashion. This provides increased insertion loss and an attenuated signal at the output port. The pair of mirrors are rotated a stepper motor or similarly controlled mechanism.
An alternate mechanical variable optical attenuator is described in U.S. Pat. No. 4,398,806 (Bennett, et al.). Bennett et al. describes a variable optical attenuator that has wedge shaped plates that are adjusted for form Fresnel lens structures. Each of plates has two surfaces defined by an angle of convergence, a pair plates are supported with two of the surfaces of each plate being spaced apart and in parallel alignment and with the angle of convergence of the two plates being in opposite directions. A second pair of plates are supported with two of the surfaces of each plate being spaced apart and in parallel alignment and with the angle of convergence the plates. One plate of each pair of plates is adjusted to modify the attenuation of the variable optical attenuator.
As the demand for communication networks has increased, wavelength division multiplexing (WDM) is becoming the technique used for increasing the amount of information that can be carried on-fiberoptic cables. Variable optical attenuators are employed within the network to allow the equalization of the gain of the bands of light frequencies transmitted on the fiberoptic cables. Further, the variable optical attenuators allow the addition and removal of selected bands or channels at various terminal points of the communication network.
Refer now to FIG. 1 for an overview of an application of a variable optical attenuator. FIG. 1 illustrates an amplification and balancing device for a fiberoptic communication channel. The light 10 from a fiberoptic cable in a wavelength division multiplexed communication system is composed of multiple wavelengths xcex1, xcex2, xcex3, . . . , xcexn. Prior to equalization and balancing each of the wavelengths xcex1, xcex2, xcex3, . . . , xcexn have non-equal amplitudes or gains. For proper operation these amplitudes or gains must be equal or balanced.
The light 10 from a fiberoptic cable is the input to an optical demultiplexer 15. The optical demultiplexer 15 separates the wavelengths xcex1, xcex2, xcex3, . . . , xcexn of the multiplexed light signal 10 into the individual light signals 20a, 20b, . . . , 20n. The individual light signals 20a, 20b, . . . , 20n are each applied respectively to variable optical attenuators 25a, 25b, . . . , 25n. The outputs of the variable optical attenuators 25a, 25b, . . . , 25n are then the inputs to the multiplexer 30. The individual light signals 20a, 20b, . . . , 20n are then recombined to form the input light signal input to the erbium-doped fiber amplifier (EDFA) 35. The erbium-doped fiber amplifier 35 amplifies the light signal to form the output light signal 40 that is transferred to a subsequent fiberoptic cable.
The variable optical attenuators 25a, 25b, . . . , 25n are each calibrated to adjust the gain of the wavelengths xcex1, xcex2, xcex3, . . . , xcexn of the individual light signals 20a, 20b, . . . , 20n such that outputs of the variable optical attenuators 25a, 25b, . . . , 25n are approximately equal. The recombined and amplified light signal 40 now has wavelengths xcex1, xcex2, xcex3, . . . , xcexn that have equal gain and are balanced.
An alternate application for variable optical attenuators is shown in FIG. 2. This application illustrates a switching application when separate wavelength channels within different light signals 110, 125, 130, and 175 are added and removed for redirection to receiving and transmitting nodes (not shown) of the communication network. Generally the light signals 110 and 130 are demultiplexed to their individual wavelength channels. Those individual wavelength channels that are to be removed from the light signals 110 and 130 are separated and transferred to the receiving nodes. The individual wavelength channels that are to be added are combined with the remaining wavelength channels to form the light signals 125 and 175. In order for the removed wavelength channels and the newly formed light signals to have appropriate balance and gain across all the wavelength channels, a variable optical attenuator is added to each channel to perform the attenuation to equalize the gain of each channel.
Referring now to FIG. 2 for a more detailed discussion. The light signals 110 and 130 are respectively the inputs to the demultiplexers 115 and 135. The demultiplexers 115 and 135 decompose the light signals 115 and 135-into their component wavelength channels. Those wavelength channels 116 that are to be transferred to the light signal 125 become a first set of inputs to the multiplexer 120. Those wavelength channels 117 that are to be removed from the decomposed light signal 110 are transferred to the set of variable optical attenuators 150. Similarly, those wavelength channels 136 that are to be transferred to the light signal 175 are transferred to the multiplexer 170 and those wavelength channels 137 that are to be removed from the decomposed light signal 130 are the inputs to the set of variable optical attenuators 150.
The variable optical attenuators 150 attenuate the wavelength channels 117 to balance and equalize the light signals to form the wavelength channels 119. The wavelength channels 119 are the inputs to the demultiplexer 140, which decomposes the wavelength channels into the individual light signals 142 that are transferred to the receiving nodes of the communication system. Similarly, the variable optical attenuators 150 attenuate the wavelength channels 137 to balance and equalize the light signals to form the wavelength channels 139. The wavelength channels 139 are the inputs to the demultiplexer 160, which decomposes the wavelength channels into the individual light signals 162 that are transferred to other receiving nodes of the communication system.
To add wavelength channels to the light signals 125, and 175 the multiplexers 145 and 155 from the transmitting nodes of the communication system receive the individually separated wavelength channels 147 and 157 respectively. The multiplexers 145 and 155 combine the wavelength channels 147 and 157 to form the light signals 149 and 159 that are then inputs to the sets variable optical attenuators 150. The variable optical attenuators 150 balance and equalize the gain of the light signals 149 and 159 to respectively form the light signals 118 and 138. The light signals 118 and 138 are then respectively inputs to the multiplexers 120 and 170. The multiplexer 120 combines the light signals 116 and 118 to form the light signal 125, which is transferred, to a fiberoptic cable for transmission to a next node of the communication network. Likewise, the multiplexer 170 combines the light signals 136 and 138 to form the light signal 175, which is transferred, to a fiberoptic cable for transmission to a next node of the communication network.
The variable optical attenuators 150 balance and equalize the gain of the light signals 149 and 159 to compensate for wavelength dependent insertion loss of the optical components of the system. Further the variable optical attenuators 150 can be placed to receive the light signals 110, 175, 125, and 130 to compensate for insertion loss in the transmission media of any of the light signals 110, 175, 125, and 130. For example if the Light signals 110 and 175 have optical components inserted into the path such as another receiver, the variable optical attenuator 150 can be applied to the light signals 110, 175, 125, and 130 to provide stable signals when the receiver is placed in the path.
FIG. 3 illustrates a variable optical attenuator control system as illustrated by U.S. Pat. No. 6,061,171 (Taylor et al.). Light signals 200a, . . . , 200d are inputs to the variable optical attenuator devices 205a, . . . , 205d. The adjustments of the attenuation of the variable optical attenuator devices 205a, . . . , 205d are communicated from the variable optical attenuator controllers 225a, . . . , 225d by the control signals 235a, . . . , 235d. Taylor et al. describes a feedback structure that allows the variable optical attenuator controllers 225a, . . . , 225d to adjust the variable optical attenuator devices 205a, . . . , 205d based on the relative power or intensity of the attenuated light signals 207a, . . . , 207d. The variable optical attenuator devices 205a, . . . , 205d attenuate the light signals 200a, . . . , 200d to form the attenuated light signals 207a, . . . , 207d, which are the input signals to the couplers 210a, . . . , 210d. 
The couplers 210a, . . . , 210d sample a portion of the attenuated light signals 207a, . . . , 207d and split the attenuated light signals 207a, . . . , 207d to light signals 218a, . . . , 218d. The light signals 218a, . . . , 218d are inputs to the optical monitor control system 220 that measures the relative magnitudes of the light signals 218a, . . . , 218d and thus the intensity of the light signals 207a, . . . , 207d. The couplers 210a, . . . , 210d transfer the remaining light signals to the fiberoptic cable 215a, . . . , 215d. 
The optical monitor control system 220 transfers the measured values of the light signals 218a, . . . , 218d as the electrical signals 223a, . . . , 223d to the variable optical attenuator controllers 225a, . . . , 225d. Each of the variable optical attenuator controllers 225a, . . . , 225d determine from the power of the light signals 218a, . . . , 218d the number of steps or amount of movement is required to adjust the attenuation factor of the variable optical attenuators 205a, . . . , 205d. 
xe2x80x9cBacklashxe2x80x9d denotes the attenuation setting accuracy when adjusting the variable optical attenuator devices 205a, . . . , 205d including adjustment made by reversing the direction of motor operation. It can be shown that the accuracy or xe2x80x9cbacklashxe2x80x9d of the system as shown can cause the variable optical attenuator devices 205a, . . . , 205d to have error in their adjustment.
The feedback from the couplers 210a, . . . , 210d, the optical monitor control system 220 and the variable optical attenuator controllers 225a,. . . , 225d can cause excessive amounts of the xe2x80x9cbacklash.xe2x80x9d
An object of this invention is to provide a variable optical adjustment system to provide adjustment to a variable optical attenuator based on its desired attenuation factor.
Another object of this invention is to provide a communication system for transmitting a desired attenuation factor for a variable optical attenuator to a variable optical attenuator controller.
Further, another object of this invention is to provide a method for calibrating a variable optical attenuator to determine and record the adjustment steps versus the attenuation factor for a variable optical attenuator.
To accomplish at least one of these and other objects, a variable optical attenuation system has at least one variable optical attenuation device that receives a light signal, attenuates the light signal, and transmits an attenuated light signal. The variable optical attenuator system further has a controller circuit in communication with the variable optical attenuation device to provide an attenuation control signal to the variable optical attenuation device to cause the variable optical attenuation device to adjust an attenuation factor of the variable optical attenuation device. The variable optical attenuator system also has a data retaining device such as an EEPROM in communication with the controller circuit. The data retaining device has a listing of attenuation factors and corresponding attenuation control signals. A communication interface such as a serial data link, within the variable optical attenuator system provides communication between the controller circuit and an external command system. The external command system indicates a desired attenuation factor and the control circuit accesses the data retaining device to retrieve the corresponding attenuation control signal. The control circuit transmits the corresponding attenuation control signal to the variable optical attenuation device, which then adjusts to assume the desired attenuation factor.
The variable optical attenuation system is calibrated to correspond the attenuation factors to the attenuation control signals and store the attenuation factors and the corresponding control signals to the data retaining device. During calibration, the communication interface is used by an external calibration system to command the control circuit to send attenuation control signals to the variable optical attenuator device to adjust the attenuation factor. The external calibration system then measures the attenuation factor that corresponds to an adjustment position set by the attenuation control signal. The measurement is then transmitted from the external calibration system to the control circuit for placement within the data retaining device.
To facilitate communication during the calibration, the communication interface operates with a set of protocols. This protocol includes a send control signal magnitude code. The send control signal magnitude code include a magnitude value that is used by the control circuit to transmit the magnitude of the control signal to cause the selected variable optical attenuator device to advance to a specified adjustment. The protocol further includes a save attenuation factor code. The external calibration system transmits the save attenuation factor to the control circuit indicating that the present setting of the variable optical attenuator device has the appended attenuation and that the control signal magnitude and the attenuation should be placed in the data retaining device. At the completion of the calibration process, the external calibration system verifies the contents of the data retaining device by sending a read attenuation factor code. The read attenuation factor code instructs the control circuit to transmit the attenuation factor for the denoted address location within the data retaining device.
The variable optical attenuation system communicates with the external command system through the communication interface using a communication protocol. The communication protocol includes a current attenuation query code that is transmitted from the external command system requesting a current attenuation factor at which the variable optical attenuation device is set. A current attenuation response code is transmitted from the controller circuit to the external command system indicating the current attenuation factor of the variable optical attenuation device. The external command system sends a select active device code to the controller circuit indicating which variable optical attenuation device is to be active. A set active device attenuation code is transmitted from the external command system to the controller circuit indicating the desired attenuation factor at which the active variable optical attenuation device is to be set. Once the variable optical attenuator device is set to a desired attenuation factor for the particular fiber communication link connected to the variable optical attenuator device, the external command system sends a store active device attenuation code to the controller circuit commanding the controller circuit to record a current attenuation factor for the active device.